1. Field of the Invention
The present invention is directed to an apparatus for transporting and for printing print media, particularly letters in postage meter machines.
2. Description of the Prior Art
In machines of the above type, the print media are conducted along a printer device and the franking and possibly further information are printed in one pass.
It is standard to conduct the print medium along the printer device either lying flat, see, for example, as in U.S. Pat. No. 5,467,709, or standing on an edge, see, for example, German OS 25 24 670 and U.S. Pat. No. 5,025,386.
In any case, it is important that the print medium and the printer device reach a defined position relative to one another so that the imprint ensues at the intended location and with adequate quality.
Given horizontal transport, a relatively large seating surface is required in conformity with the largest print medium format which may be used, resulting in a large floor area is being required machine. In the solution according to U.S. Pat. No. 5,467,709, printing is carried out in non-contacting fashion with the ink print head. The letter is conveyed clamped between a driven conveyor belt and resiliently seated pressure rollers, whereby the letter also lies against a longitudinal guide plate. The longitudinal guide plate is provided with a recess matched to the conveyor belt and with a rectangular recess for the ink print head, the nozzle rows of the ink print head proceeding diagonally across the print head. The conveyor belt, the longitudinal guide plate and the ink print head are arranged above the letter. The resilient pressure rollers and a pressure roller resiliently seated in the printing region are arranged under the letter. The spring-caused movement extent of the pressure rollers and of the pressure plate corresponds to the maximum thickness of the letters, which can fluctuate between 0.2 mm and 20 mm. The spring power must be adequate for the entire weight range of the letters--approximately 20 through 1000 g--and must also be adequate for the letters to lie in planar fashion in the region of the recess for the print head. A constant, optimally slight distance between print medium and ink print head must be adhered to for the non-contacting ink print process so that splatters have little influence and so that the print medium does not touch the nozzle surface and smears are thus avoided. The risk of smearing still exists, however, when the letter leaves the region of the recess and then glides in a constrained manner along the longitudinal guide plate. These conditions are difficult to achieve to given rapidly changing mixed mail.
A system for processing outgoing letter mail differing in weight and differing in dimensions is known from German PS 25 24 670, wherein the letters are conveyed on edge with a conveyor path. The conveyor path has driven roller pairs and a conveyor belt. In the region preceding the franking imprint, the letter is stopped by the conveyor belt by a friction belt and is transported by a guide rollers to a postage drum or printing drum with a counter-pressure drum. The counter-pressure drum is seated on a moveable pressure base whose distance from the printing drum is adjustable corresponding to the thickness of the letter currently passing through the franking station.
Aside from the fact that printing drums will no longer meet stricter postal authority demands in future, the pressure base contains a considerable number of moving component parts. This requires a correspondingly large energy outlay for overcoming the mass moment of inertia.
Further, a postage meter machine is known from U.S. Pat. No. 5,025,386, wherein the letters are conveyed upright, slightly inclined on a circulating conveyor belt. The letters thereby lie against a guide block in which a print window is provided. A thermal printing head is arranged so as to be height-displaceable and laterally displaceable in the print window, which produces a franking print on the letter. The size of the print window must be matched to the maximum length and width of the print format. The individual letter is moved up to the print window and is then arrested and pressed against the guide plate or the print window with a pressure plate. The pressure plate is driven by a motor via a tooth gearing and crank articulation. This is a comparatively great mechanical outlay and the counter-pressure forces exerted for the thermal printing are likewise large.
After printing, the letter is released and further-conveyed. Clearly, only slight letter throughput quotas can be achieved with this intermittent operation. The outlay for the adjustment of the thermal print head is substantial.
For simplifying the letter transport and for improving the printing technology, German PS 196 05 014 discloses an apparatus for printing a print medium standing on edge that allows a contact-free printing with an ink print head. Compared to the floor area of the franking and/or addressing machine, a guide plate inclined slightly relative to 90.degree. with a recess for an ink print head and a circularly running conveyor belt orthogonal to the guide plate are employed on which the print medium, particularly letters or other mailings, reside lying tilted against the guide plate, the guide plate being inclined 18.degree. out of the perpendicular. Due to the utilization of the continuous transport and of the ink print head, a continuous printing process is possible. Since printing is carried out in a non-contacting manner, the placement force is adequate for a defined attitude of the mail relative to the print head due to the inclination of the guide plate and of the conveyor belt. The friction at the guide plate can be kept low with a correspondingly smooth surface and/or slide rails. The recess which is open in the transport direction prevents a smearing of the print format and lengthens the drying time for the printed dots. If the drying time of the ink is extremely short, it is proposed to provide a correspondingly long, closed recess. If smearing of the print format occurs in the deposit of pieces of mail, this is due to the excessively high transport speed or due to lack of smear-resistance of the ink; at any rate, a drying time for the printed pixels of applied ink droplets that limits the transport speed should be optimally short. This is possible to only a limited degree on the basis of a correspondingly selected ink. Given pieces of mail that have an unstable or uneven contact, it is difficult to constantly adhere to a defined distance of the print medium surface from the nozzles of the ink jet print head.
Moreover, given a pronounced inclination of the guide plate relative to the perpendicular on the reference plane, a differential speed between a thick envelope, particularly a franking tape or a similar print medium, and the conveyor belt can arise when the friction of the envelope or print medium surface at the guide plate and/or the slide rails cannot be kept low enough compared to the friction of the envelope or print medium edge at the conveyor belt. It is obvious that such an apparatus is better suited for thick envelopes or packages or mailings having a higher inherent weight than for thin envelopes or light weight pieces of mail. This solution is unsuitable for printing address or franking tapes.
For improving the letter transport, German PS 196 05 015 discloses an apparatus for printing the print medium standing on edge that allows a non-contacting printing with an ink print head on thin print media. Specific pressure elements moveable toward and away from the guide plate are arranged on the conveyor belt as means for applying a thrust force to the print medium along the guide plate. The franking system Jet Mail.RTM. developed by the assignee on the basis of the aforementioned German Patent still allows the transport of up to 5,500 letters per hour with a thickness up to 20 mm and is thus the fastest franking system with ink jet printer technology. This solution also guarantees slip-free transport of franking tapes and even processes mixed mail. Due to the utilization of pressure elements on the conveyor belt surface, however, the latter, differing from the solution according to German PS 196 05 014, cannot be utilized in full width. This, of course, leads to irritating delays when the franking system is stopped given excessively thick letters in order to print a franking tape and then to glue this on by hand. It would therefore be desirable if even thicker letters could be automatically transported and franked.
The pressure elements also project so far from the conveyor belt surface that a position relative to the recess for the ink print head is reached. For the application of the franking imprint, a first nozzle of the ink print head must already print at a spacing of 10 mm from the placement edge at the conveyor belt surface. At least one pressure element and at least a number of nozzles thus exist for which a clamping of the piece of mail or print medium, for example a franking tape, is effective. This clamping can only be conditionally applied given specific, thin pieces of mail when, for example, the letter contains paper clips or other irregularities in this region or when an unstable envelope content does not guarantee a dimensional stability of the piece of mail. In such instances, the distance of the print medium surface from the nozzle plane of the ink print head cannot be kept constant, this contributing to a degradation of the print quality.